(1) Field of the Invention
The present invention relates to a direct-heated flow measuring apparatus having a film resistor which serves as a temperature detecting means, as well as an electric heater. Such a direct-heated flow measuring apparatus can be used, for example, for measuring the flow rate of engine intake air.
(2) Description of the Related Art
Generally, in an internal combustion engine, the amount of intake air is one of the most important parameters for controlling the fuel injection amount, ignition timing, and the like. A gas-flow measuring apparatus, i.e., an airflow meter, is provided for measuring the same. One of the more common prior art airflow meters is the vane-type, which is, however, disadvantageous in scale, response speed characteristics, and the like. Recently, airflow meters having temperature-dependent resistors have been developed, which are advantageous in scale, response speed characteristics, and the like.
There are two types of airflow meters having temperature-dependent resistors, i.e., the heater-type and direct-heated type. The heater-type airflow meter may consist of an electric heater resistor provided in an intake-air passage of an engine and two temperature-dependent resistors arranged on the upstream and downstream sides of the electric heater resistor. In this case, the temperature-dependent resistor on the downstream side is used for detecting the temperature of air heated by the heater resistor, while the temperature-dependent resistor on the upstream side is used for detecting the temperature of non-heated air. The current flowing through the heater resistor is controlled maintain a constant difference in temperature between the two temperature-dependent resistors, then, by detecting the voltage applied to the heater resistor, the mass flow rate of air can be calculated.
In this heater-type airflow meter, if no upstream temperature-dependent resistor is provided and the current of the heater resistor is controlled for a constant temperature of the downstream temperaturedependent resistor, the voltage applied to the heater resistor can be detected to determine the volume flow rate of air.
On the other hand, the direct-heated type airflow meter may consist of a film resistor which serves not only as an electric heater, but also as a temperature-detecting means for detecting the temperature of the heated air. Also, the direct-heated type airflow meter may consist of a temperature-dependent resistor for detecting the temperature of non-heated air. Thus, the current flowing through the film resistor is controlled for a constant difference in temperature between the film resistor and the temperature dependent resistor, thereby detecting the voltage applied to the film resistor as the mass flow rate of air. In this direct-heated type airflow meter, too, if no temperature-dependent resistor is provided and the current of the heater resistor is controlled for a constant temperature of the film resistor, the voltage applied to the film resistor is detected as the volume flow rate of air.
Since the film resistor of the direct-heated type airflow meter serves as a temperature-detecting means for heated air, that is, no additional temperature detecting means for heated air is necessary, the direct-heated type airflow meter is smaller in size than the heater-type airflow meter.
In the direct-heated type airflow meter, the film resistor may consist of an insulating substrate such as a ceramic substrate or monocrystalline silicon substrate, a resistance pattern of platinum (Pt), gold (Au), etc. on the insulating substrate, and a heat-resistant resin on the resistance pattern.
Usually, the response characteristics and dynamic range of the direct-heated type airflow meter are dependent upon the heat mass and adiabatic efficiency of the resistance pattern, which serves not only as a heating means but also as a temperature detecting means. In order to obtain the most excellent response characteristics and largest dynamic range, the resistance pattern should be ideally in a completely floating state in the air stream.
For this purpose, there may be suggested a direct-heated type airflow meter in which a section is cut away (below, "aperture") between the heating and temperature detecting portion of the substrate, including the film resistor, and the supporting portion of the substrate. This "throttles" the heat transfer and thus increases the adiabatic efficiency of the heating and temperature detecting portion and improves the response speed and dynamic range of the airflow meter.
As explained above, however, when throttling of heat transfer is carried out by providing an aperture between the heating and temperature detecting portion of the substrate and the supporting portion thereof, the mechanical strength of the heat transfer throttling portion of the substrate is very small, thereby weakening the airflow meter.